1. Field of the Invention
This invention relates generally to image data processing and display. More particularly, this invention relates to an apparatus and method for image data processing to accurately suppress the false color phenomena caused by different color saturation speed of image sensing elements due to the high intensity of the incident light.
2. Description of the Prior Art
As the techniques of image sensing and graphic display, aided by recent advancements made in electronics and data processing, are achieving great progresses, there are still technical difficulties in attempt to overcome the problems of false color phenomena. A false color signal is generated in a single-plate type of color imaging devices when the incident light has a high intensity. The false color phenomena is caused by the fact that the sensing elements are saturated at different incident light intensities due to the difference in the transmittance of the color filters provided in the imaging system. The false color can be generated where the incident light has high intensity or along the edges where there is a cross over between the bright and dark image zones. This problem not only causes the displayed image to appears in false colors, the image shapes may be distorted if the false colors are shown over an extended area on the screen. Therefore, in order to improve the image quality, the display of the false colors must be suppressed when the intensity of the image signals becomes too high.
A method was proposed in order to overcome this problem by selecting the transmission characteristics of the color filter in a manner such that the quantities of saturation light of the light sensing elements corresponding to each of the red (R), green (G) and cyan (Cy) color filters are identical. However because the quantity of the saturation light of each radiation sensing element changes with a change in color temperature which causes the determination of a correct saturation light quantity not achievable. Theoretically, this difficulty can be resolved by making use of a color temperature correction or compensation circuit for each color filter, however, such design and implementation will cause the image sensing and display device to become very complicate and expensive.
To solve this problem, an alterative method is used by first detecting the occurrence of a saturation output signal and then suppressing a color carrier signal. This often requires that the video signals must be first encoded in a NTSC format such that the color carrier signals can be separately processed. The application of this technique is therefore limited that it cannot be applied to the pre-recorded data stored in a magnetic recording circuit which has not been first processed by an NTSC encoder. Additional difficulties may also arise if the color suppression is performed at a color subcarrier stage. An over-suppression may often occur because the low-pass filter in a color separation unit often broaden the color signal over the time range. A range of the color signal to be suppressed will have been widened in the direction of time which may cause an error color signal to be generated due to this broadening effect.
In order to overcome the above technical difficulties, Kaji et al. discloses in U.S. Pat. No. 4,754,323 entitled "Color Image Pickup Device in Which the Level of A Sequential Color-Difference Signal is Controlled on the Basis of the Level of the Luminance Signal" (Issued on Jun. 28, 1988), a color image pickup device. The color pickup image device includes an image sensor combined with a color separation filter; a detector for detecting a brightness level of an object; a color signal forming circuit for forming a color signal from an output of the image sensor; a sequence circuit for sequencing the color signals; a controller for controlling the output of the sequence circuit responsive to the output of the brightness detector; and a modulator for modulating the output of the controller. The false color suppression circuit utilizes a low-pass filter and a high brightness detector for detecting the brightness of the image data and then comparing the brightness to a threshold saturation level. The false color suppression circuit sends a control signal to a variable gain amplifier to suppress the detected false color when the saturation threshold level is exceeded.
The technique of cancelling the false color signal generated in the high intensity component by detecting the in-phase luminance signal as disclosed by Kaji et al. may still encounter the difficulties that the false color on the periphery is left uncanceled. There are several factors which may cause this error to occur. There are differences in frequency characteristics and slight phase-difference between the luminance and color signals. A flare image may also prevent the color suppression technique to function properly. Referring to FIG. 1 wherein the intensity of a signal is shown as function of time. Generally, there is a rising portion and a falling portion of the signal. On both sides, the rising and the falling edges, the color cancellation is insufficient, e.g., when a threshold of L.sub.high is used in FIG. 1, due to the above mentioned technical limitations. For the purpose of widening the length of the false color suppression range, in one of the preferred embodiments, Kaji et al. employ a low pass filter in the process of color separation. This will extend the color suppression to both sides of the periphery in the color cancelling portion, however, in the contrary, this may cause over-suppression (See FIG. 1 where L.sub.low is applied) if the widening of the time range is not managed properly.
Various techniques of utilizing delay circuits are proposed to resolve these problems. Tusji discloses in U.S. Pat. No. 4,974,066 entitled "Circuit for Preventing High-Intensity False Color Caused By Color Separation Filters" (Issued on Nov. 27, 1990) a false color suppression circuit which generates two delayed signals to cover the front and rear edges of a chroma signal generated by processing the output signal of an image sensor with a color filter. Tusji also uses a circuit block to generate a third delayed signal to cover the interval between the two delayed signals. These delayed signals are then used to cancel the false color signals caused by high incident light. FIG. 2A shows the timing sequence of various signals including the original signal, the chrominance signal, the delay signals generated by combinations of various delay circuit block as disclosed by Tsuji, and then the suppressing signals formed by the use of all these signals.
The technique disclosed by Tsuji is able to more precisely control the time-range over which the color suppression is to be performed. However, the lengths of these delay are fixed as part of the circuit design. The time duration of a high intensity may vary dynamically from time to time and the rising and falling rates of the signal intensity may also change and become unpredictable. The method as disclosed by Tsuji is still not able to dynamically respond to these real time variations. The errors caused by over-suppression or under suppression may still occur when the dynamic variations of the high intensity image signals exceed the design ranges of these fixed length delays. One specific example is a situation when the variations of the high intensity signal have several fluctuations within a fixed delay cycle difference as that shown in FIG. 2B. A portion of the color is erroneously suppressed due to the limitation that the delay circuit does not have the capability to dynamically respond to the changes of the signal variations.
Therefore, there is still a demand in the art of image sensing and display for a new method and data processing system which can accurately and dynamically monitoring the intensity of the image signals and cancelling the false colors such that the limitations and difficulties of the prior art may be resolved. Particularly, this processing system must have a method to reduce or eliminate the errors caused by under suppression or over suppression of the image data when a high intensity image signal is detected.